Nitrided article of ferrous character containing a case-toughening element combined with a balanced case-forming element



Patented Feb. 7, 193 3 UNITED STATES WARE PATENT OFFlC E laminar mains rnmvcn, or nmzannrn, NEW mnsnr, ASSIGNOB To 'rnn INTER- Na'rmnar. menu. com an mo, ornnw roux, N. Y, A'conroaarron or DELA- NlTBlDED ABTIcLE F FEBBOUS CHARACTER CONTAINING A OASE-TOUGHENING -ELEMEN1 COMBINED WITH A BALANCEP CASE-FORMING- ELEMENT Ho Drawing, Original application filed June 19, 1931, Serial Ho. 544,044. Divided and this application filed September 6, 1832. Serial Io. 881,895.

This invention relates to improved nitrided articles of ferrous character contaln ng a case-toughening element combmed with a balanced case-formingelement and to a.

process of manufacturing such improved nitrided articles, and more particularly to an improved nitrided article constituted of steel or a ferrous alloy and containing a casetoughening element combined with a balanced casedforming element andto a process roducin such nitrided article. t is wellnown that relativel soft steels may be provided with a hard sur ace by casehardening them. Heretofore, cases have been produced by carburizing the surfaces high hardness were of the articles and then hardening them by quenching. Articles hardened in this man her. were subject to warpage or distortion which rendered some of such articles unfit for use or necessitated finishin cally after treatment. It oreover, articles which were case-hardened with carbon, could only be made.with hardness up to about 650 to 750 on the Brinell hardness scale.

For producing greater surface hardnesses in steels, nitriding was proposed. As is well known, the nitriding (nitride hardening or nitrogen hardening) processes are generally .more expensive than the conventional case hardening (carburizing and hardening) processes. Steels ofspecial composition are required (alloy steels) and in addition; the nitrided articles lacked reliability and reproducibility. Moreover, when cases of very roduced they were relatively brittle and o ten they cracked in use under comparatively low pressure or shocks in machineryparts. Although the art was Tstruggling 'to provide improvements in nitrided steels, which would better meet the demands of commercial, industrial and practical use, such im roved nitrided steels have not been provide r In the United States Patents #1,487,554

. and #1,649,398 to Fry, a process of nitriding them mechani steels and an article produced thereby are set forth which describe that aluminum is an essential element inv the steel to be nitrided.

The Fry process essentially involves the addition to steel or ferrous alloys, of aluminum. alone or aluminum in combination with chromium or the like. Phillips'in a later United States Patent #l,697,083 points out certain limitations and disadvantages of the I Fry process and product and states that F rys product is deficient in toughness and cohesivenemand that molybdenum is necessary in steel for the production of a satisfactory conimercial nitrlded steel. In addition, the Fry patent contains other defects. For exam 1e, Fry states that b the process described in his Patent (#1,48 ,554 and #1549398) marginal layers of an extraordinarily high hard ncss are acquired insteel alloys which contain 05-20% aluminum and either separate- 1y or in any desiredcombination 0.5 to 4.0%

nitriding process but .also a deleterious or unfavorable effect on the nitrided steel. The aforesaid efi'ects of nickel have been investigated by Guillet as shown in his article in enie Civil, vol. 91, pp. 38, 60, 86, 472 (1927),

by Fry as shown in his articles in Kruppsche Monatschafte, vol. p. 137 (1923) and in Stahl and Eisen 43, 127 (1923) and Sergeson as shown in his article in the Transactions of the American Society for Steel Treating,

vol. 16, p. (1929) etc.

It has now been discovered that certain metals, for instance, nickel, molybdenum, chroxmum, vanadium, heretofore considered as equivalents of each other are not equivtoughness alent and interchangeable but havedifierent and sometimes directly opposite characteristics with respect to the nitriding processes for nitriding steels andcthe products thereof; that some of the metallic elements heretofore considered necessary in steels for nitriding are unnecessary; that a substantial improvement and increase in toughness of the cases of nitrided steels may be effected; that an increased nitrogen diffusionin the article may be obtained; that the highly important hardness-toughness relations and the depth of the case produced by nitriding may be controlled; and that an improved, stable, reliable and reproducible nitriding steel can be produced by incorporating a case-forming element or several of such elements conj ointly with a case-toughening element or several such elements in certain relation to each other in a steel, ferrous alloy or other metal or alloy capable of being case-hardened by nitrlding.

By case-forming element, is meant such an element that causes the steel to take up nitrogen in its surface or marginal layers in such a manner that the hardness in the marginal layers is increased very appreciably when the steel is nitrided in an accustomed manner, such for example as by heating in an atmosphere of ammoma at suitable temperatures for suitable periods of time.

By case-toughening element, is meant such an element that causes an improvement in the toughness of the case. -The term as used in this specification represents improved ductility combined with improved strength and sometimes also combined with improved shock resistance.

According to the present discovery, the case-forming element may be an element such as chromium, aluminum, or some other element which is capable of causing in a steel or ferrous alloy, a relatively high rate of absorption of nitrogen and a relatively low rate of nitrogen diffusion, and the case-toughening element may be nickel, molybdenum or some other element which is ca able of imparting to the case on a nitri ed steel or ferrous alloy, an increased ductility combined with an -increased strength. The casetoughening element or elements are added to steel or ferrous alloy in certain proportions with respect to the case-formin g element or elements so as not only to counteract embrittling and weakening efi'ects of the caseforming element orelements in the case on the article but also to improve the structure and other physical pro erties of the case formed on the article. T e additions of casetoughening and case-forming elements may be so controlled as to improve, at the same time, the properties of the core of the article.

The elements mentioned hereinabove have as a dominant characteristic either case-forming roperties or case-toughening properties. n some instances, elements may have approximately equal case-forming and casetoughening properties. For instance, vanadium dependin upon its conditions of application inelu ing percentages present exhibits both case-forming and case-toughening properties. In percentages around 0.5% it shows marked case-forming properties.

Generally speaking, the present invention contemplates increasmg the percentage of the case-toughening element or elements as the percentage of the case-forming element or elements is increased in a steel or ferrous alloy so as to counteract certain detrimental tendencies of the case-forming element or elements as well as to effect an im rovement in the structure and'properties'o the case on the article. The percentage of the case-forming element to be added will depend upon the properties desired, such as the strength, hardness, Wear-resistance, etc., and upon the individual characteristics of the element andupon the composition of the steel or ferrous alloy to be nitrided. The percentage of the case-toughening element to be added depends upon its individual characteristics, the case-forming element and the thereof added to the metals or al oys under treatment. and the composition of .the steel or ferrous alloy to be nitrided-and upon the properties desired in the final roduct.-

In the practice of the invention, it is.preferred to use nickel and molybdenum conjointly as the case-toughening elements because not only tough cases can be provided on the article but also improved core properties not attainable by the use of either of these elements separately can be obtained. It is preferred to have the nickel addition eiual or exceed one part of nickel to one part 0 molybdenum (by weight) but the benefits obtained from the nickel and molybdenum as case-toughening elements are not restricted to these preferred proportions as is shown by the examples hereinafter described.

Generally speaking, it is-preferred to use such a percentage of case-toughenin element that the ratio of the percentage 0 casetoughening element to the percentage of the case-forming element is about 0.8 :v 1 when im proved case-toughness is desired without an ercentage appreciable decrease in maximum hardness.

Ratios of not less than about 0.8: 1.0 and not more than about 5 1 should be used, the ratio depending upon the characteristics of thecase-forming elements added and u on the 7 properties desired in the nitrided stee s. The f igher ratios of case-toughening element to case-forming element are employed in those instances where the maximum ease hardness desired becomes lower. In practice, it hasv been found that a ratio of 3: 1 gives satisfactory and useful results. When the prior art ratios of less than about 0.8: 1 are employed,

Example N 0. '1-

In accordance with the invention a steel, for instance, has incorporated therein'about 2% (by weight of case forming elements, which may be a out 1% (by weight) of aluminum and about 1% (by weight) of chromium, and about 1% (by weight) of case toughening elements, which may be about 1 (by weight) of nickel and about 4% (by wei ht) of molybdenum. The composition of t e steel may be as follows C about 0.2% (by weight) Si about 0.3% (b weight) Mn about 0.6% y weight) P about 0.02% (by weight) S about 0.03% (by weight) Cr about 1.05% (by weight) (case-forming element).

Al about 1.07% (by weight) (case-forming element) Mo about 0.24% (by weight)- (case-toughening element) Ni about 1.54% (by weight) (case-toughening element) Fe about balance The case-forming elements and the casetoughening elements may be incorporated in the steel in any convenient manner. It is preferred, however, to add the aforesaid elements to the steel'while the latter is in a molten condition. After the addition to and incorporation in the steel of said elements,

the steel may be cast into ingots or the likeand articles made therefrom. The articles may then be subjected to nitriding in any approved or suitable manner, as one skilled m the art will readily understand. Preferably, however, the articles are heated in ammonia at 900-1000 deg. F. for a customary, period of. time. Various temperatures, periods of time, etc., may be used and the following table gives some pf the customary tem ratures, periods "cycles used in nitri ing:

. The improved nitrided articles possess the propertiesshown in the followingtable. For comparative purposes, corresponding properties are tabulated of articles made from a similar steel substantially devoid of the combination of the case-f rming elements and the case-toughening elements.

Table w 353E 35? cw tough- Steels with small amounts of mess-monog fi Steel 0, s1, Mn and customary lmtron-Brlnell k purities, and (at or near n g to 7.. surface of crack an article) A.-.. No alloy additions 260 No practical c e s e t 0 crack.

13.-.- Only case-forming elements 920 215 (1% A1, 1% Cr C- Case-forming ements with 890 am lnaufllclent we toughenln g g l. r. M o

D Balanced cese-lormlng elements 890 566 and case-to bening elements g7; 91% 5% The foregoing samples were nitrided'for about 4.8 hours at about 975 F. with about 25/30% dissociation of ammonia.

The addition of the case-toughening elements (It of nickel combined with 4% molybdenum) together with 2% of caseforming elements 1% aluminum and 1% chromium) has resu ted in greatly improved case-toughness without an appreciable change in maximum case hardness as compared to a steel containing only case-forming elements (steel B Table 2) or case-forming elements with insufiicient quantities of case-toughening elements (steel C Table 2).

The'steel D of Table 2 containing nickel has a higher core hardnessand strength, as normalized, or when quenched and then tempered at 1200 F. to 1300 F. than a corresponding steel without the nickel treated in a corresponding manner and will give better support to the case when subjected to heavy pressures. In addition, the case of the nickelcontaining steel is far superior to that of the corresponding steel without nickel. It has high hardness (compare steels C and D, Table 2) and, at the same time, it is much tougher and when deformed will not break as readily as the case of the corresponding steel without nickel. a i

The nickel-containing steel is therefore especially well adapted to applications subjected to heavy loads such, for example, as

gears in automotive construction, shafts in machinery, etc.

Example No. 2 I

If the case-toughening elements, in

amounts appreciably above about 2% are added to steels containing approximately 1,%

of each of the'case-forming elements aluminum and chromium, a substantial increase in Table 8 Steels with small amounts 01C, Si, Maximum" fig? Mn and customary impurities, casehardmed b load Steel together with approximately 1% ness monoin on each of the case-formingelements, tron-Brln- Brim ball Al and Cr, and ell to crack case 12.... Case-toughening elements as fol- 890 300 lows: practically none.

F 3.53% Ni and M2}, M0 795 960 5.07% Ni and Mo 720 1140 The foregoing samples were nitrided for about 48 hrs. at about 975 F; with about 25/30% dissociation of ammonia.

The increase in toughness produced by the relatively large increases of the case-toughening element nickel is accompanied by a de crease in the maximum case hardness (observed at or near the surface of the articles,) the amount of the decreasein the hardness and the increase in the case toughness'being controllable by the amounts of the nickel added. p

The foregoing example clearly demonstrates that the present invention provides the art with a process to control the hardnesstoughness relations of the cases of nltrlded steels by means of balanced additions of caseforming elements and case-toughening elements as well as with an improved product. The hardness-toughness control feature is of special and practical importance and industrial and commercial value since the adjustment of case hardness and case toughness I by tempering subsequent to quenching, which is effective in carbon case hardened steels, is not, as one skilled in the art knows, available in the case on nitrided steels.

Y The marginal or surface layers of my improved nitrided articles will not only better resist cracking of the case but will withstand higher loads than heretofore and may be deformed further than hitherto without cracking. These features may be SllOWl'lflIlddGlllonstrated by the following table showing results of torsion and bend tests obtained upon I steels E, F and G of Table 3.

Table i k-Torsion test results on steel: E and G (specimens 34-inch diameter-8 inches long) The foregoing samples were nitrided for about 48 hours at about 975 F. with about 25/3075 dissociation oismmonia.

Each of the foregoing results is the average of two tests.

Table 5 .Bend test results on steels E, F and G (specimens ,5 max mchX'l inclws-tested on 4 inch span) Deflection Steel iii ti 0 one crack case M E (No added nickel) 835 044 F (3.5% added nickel) 2:180 .060 G (5% added nickel) 4, 675 067 The foregoing samples were nitrided for about 48 hours at about 975 with about 25/30% dissociation of ammonia.

Each of the foregoing results is the average of two tests.

Many tests in which the nitiided surfaces have been removed to various depths before testing have indicated that those torsion test results and bend test results are typical of the beneficial influences throughout the hard nllarginal layers (case) of the nitrided artic es.

E sample N0. 3

C about 0.14% (by weight) M n about 0.62% (bywei ht) S1 about 0.18% (by-Wei it) P about 0.02% g by weig t S about 0.03% by weight) Cr about 1.16% (by weight) V about 0.55% (by weight) Ni about 1.63% (by weight) Fe about balance (by weight) 4 Comparison of this steel with the corresponding one in which the case-forming elements have not been balanced against the case-toughening elements shows the marked superiority of the former. The addition of the nickel results in an appreciable increase in the case toughness without appreciable loss in the maximum case hardness.

The torsion test results which follow demonstrate the ability to control the hardness toughness relations of the marginal layers of nitrided articles through nickel additions adjusted to the case-forming elements presout.

Table 6 Torque in inchlbs. to crack case after removing the specified depth of case by grinding subsequent to mtriding Angle, d per inch to one case after removing the case subsequent to nitriding Steel 0 Cr Ni None 0.002" 0.008" None 0.002" 0.000"

14 0.49 None 1.2 2.1 2. 16 .55 1.63 2.0 5.8 6.

The steels listed in Table 6 were nitrided about 18 hours at about 975 F. with about 25/30% dissociation of ammonia. The test specimens were 4-inch diameter by 8 inches long. All results are from tests in duplicate.

Emample N 0. 4

One aspect of the invention ma be further illustrated by the followin I to a steel containing small amounts 0% carbon and the customary impurities, there is added about 1 per cent of a case-forming element, such as chromium, and insufiicient amounts of casetoughenin elements, say only about ,4; per cent of mo ybdenum, the resultant hardnesstoughness relations of the marginal layers of nitrided articles made therefrom, will not be as good as those which can be secured by balanced additions made according to this invention. In this instance, the addition of about 1 per cent nickel combined with the aforesaid alloy additions completes the proper amount of case-toughening elements to give a relatively high degree of toughness without an appreciable decrease in maximum case hardness. This illustrates one feature of the control of the hardness-toughness relations of nitrided articles. The following table illustrates the advantages of this feature of the present invention.

The foregoing samples were nitrided for about 48 hours at about 975 F. with about 25/30% dissociation of ammonia.

Example N0. 5

The efi'ectiveness and usefulness of nickel as a case-toughening element in nitrided steels and articles made therefrom, which 1mprovement constitutes one aspect of this invention, may be further illustrated by comparison of the results of tcsts of a steel containing about 2% chromium and a steel containin about 2% chromium and about 236% of nic el, viz.

Table 8 Case toughness Maximum case- Steel 0 Or Ni hardness monoggi g tron Brinell crack w 0 0.20 208 None 780 200 P .25 2.63 an l 830 52a The foregoing samples were nitrided for about 48 hours at about 900 F. with about 25/30% dissociation of ammonia.

The balanced composition of the steel P of Table 8 made according to the present invention has very much higher case-toughness without appreciable loss in maximum case hardness as compared to the steel 0 of Table 8 with unbalanced composition comprising case-forming elements without com pensating case-toughening elements.

Ewample N0. 6

As has already been set forth, one of the therefrom by balancing the additions of caseforming elements and the additions of case toughening elements. Nickel is especially effective in the group of case-ton hemng elements which may be used for t is purpose as is shown by the results of the experiments which follow. The following table shows steel Q, with no case-toughening element, and steels S and T which embody the principles of the present invention.

- Table 9 Case tough- Steels with about 1% Al together x35 22 3 ness load w sm amoun so n g. on steal an all t is s M nessmom ink and customary impurities, and mu} Brinell the following nickel additions Brine ball to crack ease Q No added case-toughening element. 805 S 3.5 added nickel 645 950 T 5.1% added nickel M0 176.8

' The foregoing samples were nitrided. for about 48 hours at about 975 F. with about 25/3070 dissociation of ammonia.

decrease in the maximum case-hardness was dependent upon the proportions of nickel added, that is to say, upon the adjustment Table 10 Case-forming elements g g g Aluminum Molybdenum Nickel Vanadium It will be observed that the present invention provides nitrided steels having controlled case-hardness and case-toughness and a process bywhich such nitrided steels may be produced. In the instances where a steel or ferrous alloy is desired with a nitrided case having relatively high hardness and a relatively good toughness, the ratio of casetoughening element to case-forming element present in the steel or ferrous alloy is about 0.8: 1. On the other hand, in the instances where a steel or ferrous alloy is desired with a nitrided case having a relatively good hardness and a relatively high toughness, higher ratios of case-toughening element to caseforming element, of theorder of about 3: 1 and about 5: 1 are used. In no case, however, is a ratio greater than about 5: 1 employed. The specific ratio in any given case will also depend partly upon the temperature and time of nitriding, carbon content of the steel or ferrous alloy, characteristics of case-toughening element or elements. of case-forming element or elements, etc. The essential feature is that the case-toughening element must be balanced or controlled with respect to the.

case-forming element.

By means of balancing (or controlling) the case-toughening element to the case-forming element as described herein, the advantages, new and/or improved results set forth in detail hereinabove may be obtained. In addition, if cases of equal depth are desired the time of nitriding may be shortened by theuse of the present process from about 5% to 25% or more. Moreover, with corresponding nit-riding procedures, it is possible to produce by use of the present invention cases deeper than those obtained in the conventional nitrided steels."

In carrying the present invention into practice, various percentages (by weight) and/or combinations of the' different casetoughening elements and case-forming elements may be used. In general, steels and ferrous alloys containing the following elements and percentages may be employed to carry the present invention into practice: aluminum 0% to about 5%; chromium 0% to about 25%; nickel 0% to about 16%; molybdenum 0% to about 12%; and vanadium 0% to about 2%. The present invention may be carried into practice with steels or ferrous alloys including cast iron, malleabilized cast iron, and substantially carbonfree iron alloys.

When iron, steel or ferrous alloy is referred to in the specification and claims, it is to be understood that the usual amounts of customary elements such as silicon, manganese, sulfur, phosphorus, etc. are present. In the claims the expression an element of the chromium type is meant to refer to caseforming elements as described herein, where as the expression an element of the nickel type is meant to refer to case-toughening elements as described herein.

The crack test applied to nitridcd cases on steels and ferrous alloy articles refers to a test in which the nitride'd case is subjected to stress as by the ball of a Brinell hardness testing machine and the case is heard to crack. In practice, the occurrence of a sharp noise or crack denotes that me chanical discontinuity, such as; a rupture, has developed in the nitrided case. By carrying the test slightly further, the crack can be confirmed by the use of a microscope, by usual examination or by a band test or by a torsion test.

It will be noted that the present invention provides an improved article of manufacture comprising a ferrous body having a nitrided case and containing a case-toughening ele ment and a case-forming element in such amounts that the ratio of the former element to the latter element is not less than about 0.8: 1 and not greater than about 5:1.

It will also be noted that the present invention provides an improved nitrided steel or the like having relatively high hardness combined with relatively high toughness in which the case has substantial depth without 1 high hardness combined with relatively high toughness, hardnesses and toughnesses of the order of those set forth in the foregoing tables are meant.

It is to be further observed that the present invention provides a new nitrided ferrous article of manufacture containing a case-toughening element and a case-forming element in certain proportions to each other. In regard to a case-toughening element, molybdenum is an equivalent of nickel. When the term nickel is used in the claims, it is to be understood that molybdenum is to be considered within the purview of the said term. Of course, it is to be understood that the same results are not necessarily secured by equal percentages of the two elements. Generally speaking, the toughening can be carried further by the use of nickel or by a combination of nickel with molybdenum than by molybdenum alone. Similarly, chromium, vanadium, and aluminum are to be considered as equivalent-cas-forming elements. When in the claims, reference is made to the term chromium or vanadium, it is to be understood that aluminum is to be considered within the purview of such terms. It is to be understood that the same results are not necessarily secured by equal percentages of the three elements. Of course, aluminum may be used alone, especially where reten tion of high case hardness at high temperatures is desired. There are often advantages in using a combination of two or more oi these case-forming elements. For example, chromium and aluminum may be used with satisfactory results.

While specific reference has been made to treatment in ammonia, the described benefits resulting from the employment of the principles of the present invention, processes and products described, are not limited as to the sources of the nitrogen which is absorbed in the marginal layers of the steel to form a case.

The present application is a division of my copending application, U. S. Serial No. 544,- 044, filed June 12, 1931.

I claim:

1. An article of manufacture comprising a ferrous body having a tough nitrided case, as nitrided, and containing nickel acting as a case-toughening element combined with aluminum acting as a ba lanced'case-forming element, the ratio of said case-toughening element to said case-forming element being not less than about 0.8 :1 and not greater than about 5: 1, and the case-forming element being present to an extent of more than about 1.5% and less than about 41% and the casetoughening element being present to an extent less than about 6 o.

2. An article of manufacture comprising a ferrous body having a tough nitrided case, as nitrided, and containing about 1.5% to about 5.0% of nickel acting as a case-toughening element balanced with a material per cent to about 1% of aluminum acting as caseforming elements, the ratio of said casetoughening element to said case-forming element being not less than about'0.8: 1 and not greater than about 5: 1, and the case-forming element being present to ail-extent of more than about 1.5% and less than about 5% and the case-toughening element being present to an extent less {than about 6%.-

3. An article of manufacture comprising a ferrous body having a tough nitrided case, as nitrided, and containing about 2% to about 5% of nickel and a material percent to about 0.5% of molybdenum actingas casetoughening elements balanced with a material percent to not more than about 5% of aluminum acting as case-forming elements, the ratio of said case-toughening elements to said case-forming element being not less than about 0.8: 1 and not greater than about 5: 1, and the case-forming element being present to an extent of more than about 1.5% and less than about 5% and the case-toughening elements being present to an cxtent lessthan about 6%.

Intestimony whereof, I have hereunto set my hand.

HERBERT JAMES FRENCH. 

